Abstract:

Much of the observed wintertime increase of mortality in temperate regions is attributed to seasonal influenza. A recent
reanalysis of laboratory experiments indicates that absolute humidity strongly modulates the airborne survival and
transmission of the influenza virus. Here, we extend these findings to the human population level, showing that the onset of
increased wintertime influenza-related mortality in the United States is associated with anomalously low absolute humidity
levels during the prior weeks. We then use an epidemiological model, in which observed absolute humidity conditions
temper influenza transmission rates, to successfully simulate the seasonal cycle of observed influenza-related mortality. The
model results indicate that direct modulation of influenza transmissibility by absolute humidity alone is sufficient to
produce this observed seasonality. These findings provide epidemiological support for the hypothesis that absolute
humidity drives seasonal variations of influenza transmission in temperate regions.